1. Field of the Invention
The present invention relates to a light-emitting device using a LED, and more particularly to a light-emitting device with improved uniformity and brightness of illumination light.
2. Description of the Related Art
Typical light sources such as incandescent lamps, mercury vapor lamps, and fluorescent lamps are light-emitting devices that convert electricity into light. These light sources have the following drawbacks. Namely, incandescent lamps generate a large amount of heat during light emission, mercury vapor lamps have a high power consumption due to a high voltage applied thereto, and fluorescent lamps cause a great environmental load since they are filled with heavy metals such as mercury.
Global warming has recently become a serious problem, and the decrease in energy consumption and reduction of carbon dioxide emission are the tasks of global importance. Accordingly, measures for protecting environment, such as reduction of energy consumption, are also important in the field of light-emitting devices. Light-emitting devices using a LED as a light source have been suggested to meet this need (for example, Japanese Patent Application Laid-open No. 2007-59163).
By contrast with incandescent lamps, LED generates little heat in the course of light emission and has a very low level of power consumption of several tens of milliamperes per unit. Moreover, LED demonstrates a long service life of not less than 70,000 hours. Therefore, light-emitting devices using LED are expected to be effective means for protecting environment.
However, although LED has the above-described merits, since it is a point light source, a sufficient quantity of light cannot be obtained with one unit. For this reason, it was difficult to ensure light intensity over a wide illuminating surface area with a light-emitting device using a LED. Accordingly, high-brightness white LED (power white LED) has been recently developed, which can be operated at a low voltage level and also ensure good visibility. Further, in order to obtain a sufficient luminous flux, various techniques have been suggested, such as a technique using a multiple integrated arrangement in which a large number of LED are arranged in a plane, and a technique using optical designs in which a reflector and an optical lens are arranged (Japanese Patent Applications Laid-open Nos. 2006-171256 and H03-159362).
However, the aforementioned light-emitting devices using LED have been pointed out to have the following problems. Thus, the development of power white LED and optical designs including optical lenses and the like are aimed at ensuring the quantity of light, but according to the present trend toward light-emitting devices of more larger size, it is desirable to ensure more larger quantity of light and enable uniform light emission over a wide illuminating surface area.
Further, in the light-emitting device in which a large number of LED are integrated, or light-emitting devices based on an optical design including optical lenses or the like, the device configuration easily becomes complex, thereby causing increase in cost. Further, power white LED demonstrating sufficiently high brightness are expensive. As described above, light-emitting devices using LED are expected to be effective means for protecting environment, and suitability thereof for industrial applications has to be increased to enable wide spread of such devices. Thus, problems associated with light-emitting devices include not only ensuring sufficient quantity of light and attaining light uniformity, but also minimizing production cost.
Further, as the device configuration becomes more complex, light emission characteristic is often degraded. More specifically, when an optical lens is formed by molding to obtain a lens effect, since the lens covers a LED, heat is emitted by the LED and the light emission characteristic of the LED becomes unstable due to this heat. As a result, the service life of LED can be shortened.